ENGINE CONTROLS - SELF-DIAGNOSTICS - 4.3L -2001 GMC Sierra 1500
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Diagnostic Procedures
1. Perform powertrain diagnostic system check. See POWERTRAIN DIAGNOSTIC SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM. After performing powertrain diagnostic system
check, go to next step.
2. If DTCs P1635 and P1639 also set, repair these DTCs first. See DTC P1635: 5-VOLT REFERENCE 1 CIRCUIT and DTC P1639: 5-VOLT REFERENCE 2 CIRCUIT. If DTCs
P1635 and P1639 are not also set, go to next step.
NOTE: Ignition must be off for a minimum of 15 seconds when you are instructed to turn ignition off. If ignition switch is not in the OFF
position for 15 seconds, misdiagnosis occurs. This information applies to every step that instructs you to turn ignition off.
3.
Turn ignition off for 15 seconds. Turn ignition on, engine off. Using scan tool, monitor fuel tank pressure sensor voltage. If fuel tank pressure sensor voltage is more than 4.3 volts, go to step 5. If
fuel tank pressure sensor voltage is not more than 4.3 volts, go to next step.
4. Turn ignition on, engine off. Review FREEZE FRAME/FAILURE RECORDS data for this DTC and observe parameters. Turn ignition off for 15 seconds. Idle engine. Operate vehicle within code
enable criteria, and as close to FREEZE FRAME/FAILURE RECORDS data as possible. Using scan tool, observe this DTC. If scan tool indicates this DTC failed this ignition cycle, go to next
step. If scan tool does not indicate this DTC failed this ignition cycle, see DIAGNOSTIC AIDS. 5. Turn ignition off for 15 seconds. Disconnect fuel tank pressure sensor connector at fuel tank. Turn
ignition on, engine off. Using scan tool, observe fuel tank pressure sensor voltage. If fuel tank pressure sensor voltage is less than one volt, go to next step. If fuel tank pressure sensor voltage is
not less than one volt, go to step 7.
6. Using a test light connected to battery voltage, probe fuel tank pressure sensor connector terminal "A" (Black wire). See Fig. 26 or Fig. 27. If test light is on, go to step 11. If test light is off, go to
step 8.
7. Using DVOM, measure voltage between fuel tank pressure sensor connector terminal "B" (Dark Green wire) and ground. If voltage is about zero volts, go to step 14. If voltage is not about zero
volts, go to step 9.
8. Disconnect PCM Blue connector C1. Measure resistance of Black wire between PCM and fuel tank pressure sensor. If resistance is less than 5 ohms, go to step 12. If resistance is not less than 5
ohms, go to step 10.
9. Repair short to voltage in Dark Green wire between PCM and fuel tank pressure sensor. After repairs, go to step 15.
10. Repair open or high resistance in Black wire between PCM and fuel tank pressure sensor. After repairs, go to step 15.
11. Check fuel tank pressure sensor connector for poor connections. If problem was found, repair as necessary. After repairs, go to step 15. If problem was not found, go to step 13.
12. Check PCM connectors for poor connections. If problem was found, repair as necessary. After repairs, go to step 15. If problem was not found, go to step 14.
13. Replace fuel tank pressure sensor. After repairs, go to step 15.
14. Replace PCM. Program PCM. See POWERTRAIN CONTROL MODULE under PROGRAMMING. After repairs, go to next step.

15.
Using scan tool, clear DTCs. Turn ignition off for 30 seconds. Start engine. Operate vehicle
within code enable criteria. If DTC runs and passes, go to next step. If DTC does not run and pass, go to step 2.
16. Using scan tool, observe stored information. If scan tool displays any DTCs that have not been diagnosed, see DIAGNOSTIC TROUBLE CODE DEFINITIONS. If scan tool does not
display any DTCs, system is okay.